The present invention relates to a heating element type mass air flow sensor and an internal combustion engine using the sensor, and especially to a heating element type mass air flow sensor suitable for measuring the amount of intake air taken into an internal combustion engine and an internal combustion engine using the sensor.
A heating element type mass air flow sensor has a feature such that it can directly measure a mass air flow-rate, and has been the mainstream sensor for measuring the flow-rate of intake air taken into an internal combustion engine used for a car. Particularly, a heating element type mass air flow sensor using a thin film type detection element fabricated by a semiconductor micro-machining technique has recently attracted due to its low fabrication cost and low power consumption, and an example of such a sensor is disclosed in Japanese Patent Application Laid-Open Hei 10-253414. This document discloses a heating element type mass air flow sensor using a so-called temperature difference method, in which the mass air flow-rate on the sensor is measured from the temperature difference between two thin film-type resistance temperature-sensing elements of the same size, separately located upstream and downstream of a thin film-type heating resistor. This conventional technique is explained below with reference to FIG. 11 and FIG. 12.
Here, FIG. 11 shows a plan view of a sensor element, and FIG. 12 shows a vertical view of the sensor at the x-axis line indicated in FIG. 11. In these figures, reference numbers 2, 3, and 4, indicate a semiconductor substrate, a diaphragm portion, and a heating resistor, respectively. Further, reference numbers 6a and 6c, and 9, indicate resistance temperature-sensing elements, and a cavity portion, respectively. Furthermore, respective reference numbers 12b, 12c, and 12f, and 24, indicate terminal electrodes, and dummy patterned resistors. Meanwhile, the x-axis indicates the air flow direction, and the longitudinal direction of the heating resistor 4 is the y-axis direction.
The cavity portion 9 is shaped on the one side of the semiconductor substrate 2, and the diaphragm portion 3 is formed on the other side of the substrate 2, so as to seal the diaphragm 3. Further, the heating resistor 4 is formed on the diaphragm portion 3, the one element 6a of the paired resistance temperature-sensing elements is formed upstream of the heating resistor 4, and the other one element 6b is formed downstream of the heating resistor 4.
Here, the terminal electrodes 12b, 12c, and 12f, are lead-connection parts of the respective resistor and sensing elements, and the dummy patterned resistors are located so to improve the temperature distribution in the longitudinal direction of the heating resistor 4.
The heating resistor 4 is powered up during the measurement of mass air flow-rate by performing a feed-back control for the turning-on of electricity in the resistor 4, so as to keep the temperature of the resistor 4 higher by a predetermined difference than that of the measured air.
Under these conditions, the mass air flow-rate is measured by comparing the two resistance values of the paired resistance temperature-sensing elements 6a and 6c, located upstream and downstream of the heating resistor 4, respectively, and this measurement manner is the pedigree of the temperature difference method.
Since the resistance values of these resistance temperature-sensing elements 6a and 6c have the temperature dependency, these resistance values are determined by the temperature of the diaphragm 3 on the cavity portion 9. Further, the temperature distribution of the diaphragm 3 depends on the heat generated by the heating resistor 4, and the mass flow-rate flowing on the sensor.
First, when there is no air flow on the sensor, the temperature distribution of the diaphragm 3, caused by the heating resistor 4, is symmetric upstream and downstream of the resistor 4, with respect to the y-axis perpendicular to the air flow direction. Accordingly, there is no difference created between the temperature of the sensing element 6aand that of the sensing element 6c. 
On the other hand, when air flows onto the sensor in the x-axis direction, the cooling effect at the sensing element 6a located upstream of the resistor 4, is larger than that at the sensing element 6c, located downstream of the resistor 4. Therefore, a difference, subject to the mass flow-rate of air flowing on the sensor, is created between the temperature of the sensing element 6a, and that of the sensing element 6c. 
This temperature difference is detected as a difference between the resistance values of the sensing elements 6a and 6c. Further, when there is no difference between the resistance values of the two sensing elements 6a and 6b, the mass flow-rate of air is set to 0, and is measured by detecting the difference of the resistance values.
However, in the above conventional technique, the temperature distribution, in the direction perpendicular to the measured-air flow direction, of the heating resistor, is not sufficiently taken into account, which in turn causes a problem as per assuring measurement accuracy.
In sensors which use the temperature difference method, although the mass flow-rate of air is measured by detecting the changes in the temperature in the air flow direction, the temperature distribution in the direction perpendicular to the measured air flow direction (the longitudinal direction of the heating resistor) greatly affects on the measuring accuracy.
However, in the conventional technique, the effect of the above temperature distribution in the longitudinal direction of the heating resistor is not sufficiently considered, and the above problem is caused.
Here, although the dummy patterned resister 24 located at one side of each resistance temperature-sensing element contributes the uniform temperature distribution, the shapes of the heating resister 4, and the sensing elements 6a and 6c, themselves, are not specifically taken into account.
The shape of the heating resistor 4 in the conventional technique is symmetric with respect to the y-axis as shown in Figure. However, since the terminal electrodes 12b, 12c, and 12f, of the respective resistors, are located at one side of the respective resistors, the shape patterns of these resistors is asymmetric with respect to the x-axis.
Therefore, when the heating resistor 4 is powered up, heat is generated at the terminal electrode (the lead-connection part) 12c, which in turn shifts the peak in the temperature distribution of the heating resistor 4 toward the lead-connection part 12c in they-axis direction, as shown by the solid line 25 in FIG. 13.
As explained above, since a shift (xcex94L) of the peak in the temperature distribution with respect to the central point in the length (L1) of the heating resistor 4 is caused, a distortion of the temperature distribution in the range of the length (L2) of the resistance temperature-sensing elements 6a and 6c occurs, and this causes an error in measuring the mass air flow-rate (in measuring the temperature difference between the elements 6a and 6c).
If the x-axis direction is inclined to the air flow direction by the variation in the attachment position of the mass air flow sensor, this error in measuring the mass air flow-rate becomes remarkable, and the shift (xcex94L) of the peak in the temperature distribution also becomes large, which in turn may shift the peak out of the effective regions of the respective sensing elements 6a and 6c. 
Moreover, in the conventional technique, since the lead-connection parts 12b and 12f of the sensing elements 6a and 6c are made of the same material (with the same resistance values) as that (those values) of the sensing elements 6a and 6c, the resistance values of these lead-connection parts 12b and 12f have a temperature dependency. To top it all off, since these lead-connection parts 12b and 12f are located on one side of these sensing elements 6a and 6c, the temperature distribution in each of these sensing elements 6a and 6c, as well as the heating resistor 4, becomes asymmetric with respect to the x-axis, and these sensing element 6a and 6c also increase the error in measuring the mass air flow-rate.
An objective of the present invention is to provide a heating element type mass air flow sensor in which the measurement accuracy is sufficiently improved by properly adjusting the temperature distribution in the mass air flow sensor.
To achieve the above objective, the present invention provide a heating element type mass air flow sensor for measuring air flow-rate by using at least a heating resistor, situated on one side surface of an electrical insulation layer formed on a semiconductor substrate, covering a cavity portion shaped in the semiconductor substrate, wherein the heating resistor is, substantially in a straight line, located in the direction substantially perpendicular to the direction of an air flow to be measured; slits are shaped in the heating resistor; and current is passed through the heating resistor so as to heat the heating resistor.
Further, in the above heating element type mass air flow sensor, the heating resistor has a structure such as that the density of current flowing in the respective areas, in which the slits are shaped, is higher than the density of current flowing other areas of the heating resistor.
Furthermore, the present invention provides a heating element type mass air flow sensor for measuring air flow-rate by using at least a heating resistor, situated on one side surface of an electrical insulation layer formed on a semiconductor substrate, covering a cavity portion shaped in the semiconductor substrate, wherein a region in the electrical insulation thin-layer, the region just covering the cavity portion in the semiconductor substrate, is substantially a rectangle with the size w in the direction of the air flow, and the size D in the direction perpendicular to the air flow; and those sizes should satisfy the inequality: D greater than 4W.
Also, the present invention provides a heating element type mass air flow sensor for measuring air flow-rate by using; at least a heating resistor, situated on one side surface of an electrical insulation layer formed on a semiconductor substrate, covering a cavity portion shaped in the semiconductor substrate; and at least one pair of resistance temperature-sensing elements separately arranged upstream and downstream of the heating resistor, respectively, on the electrical insulation thin-layer; wherein a region of the electrical insulation thin-layer, the region just covering the cavity portion in the semiconductor substrate, is substantially a rectangle with the size w in the direction of the air flow, and the size D in the direction perpendicular to the air flow, and those sizes should satisfy the inequality: D greater than 4W; the heating resistor has the length L1, and the length L1 and the size D should satisfy the inequality: D greater than 1.1xc3x97L1; and each resistance temperature-sensing element has the length L2, and the length L2 and the length L1 should satisfy the inequality: L1 greater than 1.05xc3x97L2.